A piezoelectric transformer that can be reduced in size and thickness and can bring high efficiency has been used, instead of an electromagnetic transformer, as a booster transformer for driving a cold cathode tube that is a backlight of a liquid crystal panel. The piezoelectric transformer is generally a voltage converting element, which utilizes the piezoelectric effect of a piezoelectric element for generating mechanical oscillations and taking out from its secondary electrode side a voltage amplified depending on a voltage step-up ratio determined by the shape of the piezoelectric transformer.
FIG. 40 shows a general characteristic of a piezoelectric transformer. In the piezoelectric transformer, an output voltage at its secondary side varies in accordance with a frequency of an AC voltage inputted to its primary side since the piezoelectric transformer has a resonance characteristic. Due to the aforementioned characteristic, the next form is popular as a method for controlling the brightness of the backlight at a constant level. Specifically, this method is such that the frequency of the AC voltage for driving the piezoelectric transformer is controlled to obtain an amplitude of a desired level at the secondary side, to thereby apply a stable voltage to the cold cathode tube. A linear inclined portion in the frequency area higher than the resonance frequency is utilized for the control.
FIG. 41 shows a general configuration of a backlight driving means. In FIG. 41, numeral 1 designates a piezoelectric transformer, 2 a coil, 3 a switching transistor (FET (Field Effect Transistor)), 4 a cold cathode tube, 5 a current detector for detecting electric current flowing through the cold cathode tube 4 and 6 a driving pulse generating circuit that compares the detected voltage by the current detector 5 with a reference voltage for producing a driving pulse of a frequency according to its difference.
FIG. 42 shows waveforms at each means of the circuit shown in FIG. 41. In FIG. 42, a voltage A is an output voltage waveform (driving pulse) of the driving pulse generating circuit 6, a voltage B is an input voltage waveform of the piezoelectric transformer 1 and a voltage C is an output voltage waveform of the piezoelectric transformer 1. The voltage C is given to the cold cathode tube 4 so that electric current (tube current) flows through the cold cathode tube 4. The current flowing through the cold cathode tube 4 is detected by the current detector 5. Although the input voltage of the piezoelectric transformer 1 is desirably a sinusoidal voltage, the following explanation is made with a half-wave sinusoidal voltage.
The coil 2 and the piezoelectric transformer 1 constitute a resonance circuit. Its resonance frequency is determined by the inductance value of the coil 2 and the input capacity value of the piezoelectric transformer 1. Applying energy to the resonance circuit via the FET 3 brings a half-wave sinusoidal voltage with the above-mentioned resonance frequency (voltage B). Only the resonance frequency component possessed by the piezoelectric transformer 1 itself is drawn out from the output of the piezoelectric transformer 1 to which the voltage B is inputted, with the result that the sinusoidal voltage that is the voltage C shown in FIG. 42 is outputted. This sinusoidal voltage C has the amplitude varying in accordance with the frequency of the voltage B due to the characteristic shown in FIG. 40.
Utilizing this characteristic, the driving pulse generating circuit 6 compares the voltage obtained from the current detector 5 with the reference voltage for controlling the frequency of the output voltage from the driving pulse generating circuit 6 such that the tube current is made to be a desired value. Voltages A1 to C1 in FIG. 42 respectively show waveforms of each means when the frequency of the driving pulse is lowered. The lower the frequency becomes, the greater the amplitude of the sinusoidal voltage becomes.
As described above, a method for controlling the tube current by varying the frequency of the driving pulse is generally used for controlling the brightness of the cold cathode tube 4 in the driving circuit of the piezoelectric transformer 1.
Conventionally, a circuit for realizing the above-mentioned technique has generally been formed of an analog circuit. Accordingly, there is a problem that the number of components is increased and the space for the components is large. Particularly, it has been desired that the space for the driving circuit is saved since miniaturization of equipment is important in a portable imaging equipment such as a VTR integrated with a camera or a digital camera.
In view of this, an attempt has been made for saving a space for the driving circuit and also reducing cost by digitizing the driving circuit to make one chip with the other LSIs (for example, liquid crystal controller or the like) (see Japanese Unexamined Patent Publication No. 2000-133485 (document 1)).
Several 100 MHz to several GHz clocks are normally required in order to obtain practical dimming resolution with the driving of the piezoelectric transformer of a digital system. The technique in the document 1 is a system in which a frequency of the driving pulse is distributed. This system enables a dimming resolution by a frequency control with approximately 10 MHz clocks.
The technique disclosed in the document 1 will briefly be explained. In the case of digitally producing a driving pulse, a frequency resolution required for the dimming control cannot be obtained in a method for obtaining a driving pulse by simply dividing clocks. A sharpness Q of the resonance of the piezoelectric transformer is extremely great. In case where the driving frequency of the piezoelectric transformer is about 100 KHz, for example, the frequency resolution required for the dimming control is at intervals of several 10 Hz in the vicinity of 100 KHz. However, in case where the clock frequency is 10 MHz and the driving pulse of 100 KHz is produced by using this clock, for example, the frequency resolution of only about 1 KHz can be obtained in the vicinity of 100 KHz, so that it is difficult to realize not only dimming performance but also stable lighting.
In view of this, a configuration is made such that a frequency dividing ratio is distributed in a predetermined driving cycle for obtaining a resolution with an average frequency as shown in FIG. 43. By this method, when the frequency dividing ratio is distributed in 100 cycles, for example, a hundred-fold frequency resolution can be obtained. This method enables to incorporate a cold cathode tube driving device in a digital LSI, thereby being capable of reducing a space and cost.
The driving method disclosed in the document 1 is a system for performing the dimming control by controlling the tube current. However, this method for performing the dimming control by controlling the tube current has the following problems:
(1) The current flowing through the cold cathode tube is unstable in the area where the tube current is small, so that the dimming level cannot be greatly reduced.
(2) A luminous efficiency of the cold cathode tube varies depending upon a current value, so that the luminous efficiency greatly changes in accordance with the dimming level in the tube current control.
(3) In order to change the tube current, the driving frequency is required to be changed. However, the efficiency of the piezoelectric transformer changes depending upon the driving frequency, so that there is a limitation to obtain a high efficiency within a wide dimming range.
As a means for solving these problems, there has been known a technique (hereinafter referred to as a burst dimming) in which the driving is intermittently turned on and off at a frequency (for example, about 100 Hz) that does not cause a brightness flicker. The dimming level is changed by changing a duty ratio of turning-on and turning-off. This technique has widely been used for not only a backlight driving circuit utilizing a piezoelectric transformer but also a backlight driving circuit utilizing a conventional electromagnetic transformer.
There are roughly the following two systems for realizing this burst dimming in the backlight driving circuit utilizing the piezoelectric transformer. One of them is a method for turning on and turning off a circuit power source, while the other one is a method for turning on and turning off only the driving pulse with the circuit actuated.
The former method can be adopted to the conventional circuit. In this method, a starting control upon turning on a power source and a normal control upon lighting are repeatedly executed. However, a shield circuit for turning on and turning off the power source is required in a power source line. An electric loss occurs due to this shield circuit, thus not preferable in a portable equipment in particular.
On the other hand, the latter method requires a means for turning off the detection of the tube current upon turning off the driving and holding the driving frequency at a predetermined value in addition to the conventional circuit. In this case, there is no electric loss in the power source line, thus excellent in the use of a portable equipment in particular. A technique for realizing this method in an analog system is disclosed, for example, in Japanese Unexamined Patent Publication No. 11-298060 (1999) (document 2).
The technique disclosed in the document 2 will briefly be explained. FIG. 44 is a view simplifying the technique disclosed in the document 2. It is featured in that a voltage at the current detector 5 is held by a sample holding means 8 upon turning off the driving, thereby holding the oscillation frequency of a voltage control oscillation circuit 12 at a constant level and promptly starting the lighting upon restarting the driving.
As described above, the burst dimming driving system can be realized by adding a simple circuit in the analog driving system. However, in case where the burst dimming is realized in the digital system by a frequency distribution system, there arises the following subjects:
(1) The frequency distribution system is configured such that a frequency of a driving pulse is distributed in a predetermined cycle to obtain a frequency resolution with an average frequency. It performs a current control with the distribution cycle as one set. Therefore, in case where the ON/OFF duty ratio is optionally changed, a current detection that is correct on principle of the distribution system cannot be executed, and hence, the current control is remarkably unstable.
(2) In case where the duty ratio is changed with a distribution cycle unit without destroying the distribution principle as shown in FIG. 45, a set of the distribution cycle is required in accordance with the dimming resolution in order to obtain a required dimming resolution. By this, the frequency of the burst cycle is remarkably reduced and brightness flicker is increased. Additionally, when the burst period rises high to a level that can suppress the brightness flicker, the dimming resolution cannot be obtained.
Further, there is a subject concerning a starting control upon the burst dimming (especially in case where the aimed brightness level is small) as a common subject of analog and digital systems. It is desirable to light up with a desired brightness upon lighting. However, this has been difficult in the conventional driving circuit because of the following reasons.
There are following two methods for lighting with a desired brightness. The first one is a method for performing with the burst driving upon the starting similar to the case upon the lighting as shown in FIG. 46. This method does not require extra circuits since the burst control is adopted upon starting and lighting. Further, the brightness level is shifted to the aimed brightness level without sense of a incongruity, thereby being capable of obtaining a satisfactory lighting quality.
However, the lighting performance of the cold cathode tube is deteriorated in this method, especially in case where the aimed brightness level is low.
A cold cathode tube generally has reduced lighting performance at a low temperature or under a dark ambient illuminance. This problem has been known to be improved by rising the starting voltage or lengthening time for applying the starting voltage.
However, the starting with the burst driving requires an extra time for applying the starting voltage, thereby taking time to light.
Additionally, in order to prevent extra stress due to high pressure being placed on the piezoelectric transformer upon starting, a protection function is provided to terminate the starting unless lighting is executed even after a predetermined period. There may be the case where lighting is not executed due to the influence of this protection function.
There is a method for rising the starting voltage upon the burst driving. However, this causes further stress on the piezoelectric transformer, thereby deteriorating reliability. Further, this method requires an extra circuit space to reinforce a measure for preventing a discharge caused by a high pressure, thus not preferable.
The second method is that, as shown in FIG. 47, continuous driving is performed upon the starting in view of the lighting performance of the cold cathode tube, and then, the driving is promptly shifted to the burst driving after the lighting. This method requires a lighting detecting means and a control switching means since the control is different between upon the starting and upon the lighting. If an ideal lighting detecting means can be obtained, it enables the lighting with the aimed brightness level without deteriorating the lighting performance of the cold cathode tube.
However, the conventional lighting detecting means is slow in detecting the lighting, resulting in that the brightness is greatly changed by the time of reaching the aimed brightness level as shown in FIG. 47, that causes a problem concerning the lighting quality.
The present invention is accomplished in view of the above-mentioned problems, and an object thereof is to firstly propose a driving system realizing a burst dimming in a digital system, and to realize reduced cost and saved space in a cold cathode tube driving device that can perform a burst dimming.
Further, another object of the present invention is secondly to provide a starting system excellent in lighting performance even in case where the aimed brightness level is low.
The other objects, features and advantages of the present invention will be apparent from the following description.